It is known in a power train for a motor vehicle to have a prime mover (such as an internal combustion engine), a transmission including a gear train which transmits torque to the wheels of the vehicle, a torque transmitting system (such as a clutch) between the engine and the gear train, a means for operating the gear train such as an electronic control unit, and one or more actuators which operate the clutch and/or the gear train in response to signals from the control unit.
It is further known to operate an automated gear train by a control unit which receives signals from one or more sensors, electronic circuits and/or other monitoring means and transmits signals to one or more actuators which directly actuate or operate the gear train to select a particular gear ratio and/or to shift into a selected gear. The same actuator or additional actuators can be used as a means for automatically selecting the torque to be transmitted by an automated clutch or other automated torque transmitting system between a prime mover and the input element of the gearbox in the power train. The connection between one or more actuators and the actuated part(s) of a gear train or clutch can include one or more driving units for selecting a given gear ratio and shifting into the selected gear.
There is a need in the art for actuators for various engine mounted applications that meet high peak and hold torque requirements. It is known to achieve high peak torque by providing various motor and gear train enhancements that also meet power consumption, overall size, speed and back drive requirements. While there are various known methods for achieving high peak torque, holding the high peak torque level is much more difficult.
One known method to hold high torque is to hold the high torque with a high current level. This method assumes using an actuator that meets the peak torque, size, speed, and back-drivability requirements. However, the actuator will not survive for the duration of the hold time requirement (up to ten minutes). Further, the estimated 6 to 8 amperes of current required will overheat the motor coils or electronics.
Another known method to provide high hold torque is to increase the torque capability by increasing the motor size. This approach reduces the hold current to a manageable level, but increases the size and mass of the motor beyond overall package requirements. An additional disadvantage is that, generally, motor cost increases with motor size. As motor size increases, motor current draw also increases. Such increased current draw causes thermal problems for the actuator, for the drive electronics, and for the motor. This approach further provides the disadvantage of reduced output shaft speed.
Yet another known method to hold high torque is to increase the gear ratio. While increasing the gear ratio reduces the hold current to a manageable level, such an approach presents several disadvantages. First, this approach results in reduced output shaft speed. Second, known spur gear designs must increase in size beyond package limits and further have reduced back-drivability. Third, known worm gear designs are not back drivable.
What is needed in the art is an apparatus and method to achieve and hold a high torque level while maintaining acceptable power levels, size, speed, and back drive ability targets.